This invention relates to an improved process of preparing Bi--Sr--Ca--Cu--O (BSCCO) powders, and more particularly, to a process for preparing BSCCO powders that utilize freeze-drying.
The best superconductors are those which are of single phase material rather than a variety of different compounds. However, it is very difficult to prepare the phase pure materials such as BSCCO (Bi--Sr--Ca--Cu--O). Chemical methods such as coprecipitation often result in sequential precipitation while sol-gel processing leads to carbonate formation due to the presence of citric acid or acetic acid. The freeze-drying of nitrate solutions has also been reported.
Since the discovery of superconductivity in the BSCCO system, much effort has been focused on the synthesis of the superconducting phases of BSCCO, in particular, the Bi.sub.2 Sr.sub.2 Ca.sub.2 Cu.sub.3 O.sub.x (2223) phase with a superconducting transition temperature (T.sub.c) of 110K. It is difficult to produce the 2223 phase without the partial substitution of Pb for Bi to promote the formation and stabilization of the 2223 phase. The formation of the 2223 phase is promoted by synthesis in reduced oxygen partial pressure and modification of the initial composition with excess Ca and Cu. Production of a material with a high volume fraction of the 2223 phase by conventional solid-state processing requires very long heat treating times, on the order of 150 to 300 hours (i.e., about 1 to 2 weeks). Solid-state reaction techniques commonly employed in the synthesis of these compounds have several disadvantages for multicomponent oxide systems such as (BiPb).sub.2 Sr.sub.2 Ca.sub.2 Cu.sub.3 O.sub.x. Such techniques do not yield chemically homogeneous precursors, and the rate of 2223 phase formation is slow due to transport limitations in the solid state. A further disadvantage arises from segregation due to the slow decomposition of SrCO.sub.3 and CaCO.sub.3 when these are used as starting materials.
Chemical methods such as coprecipitation often result in a sequential precipitation of the elemental compounds serving only to reduce the particle size of the same components used in the solid state techniques. Such processes typically employ oxalic acid, the presence of which could lead to the formation of SrCO.sub.3 and CaCO.sub.3. Sol-gel processing leads to similar problems of carbonate formation due to the presence of citric acid or acetic acid.
To overcome problems of chemical inhomogeneities in the precursor material, as well as the presence of carbonaceous anions, freeze-drying has been applied to the synthesis of YBa2Cu.sub.3 O.sub.x and has recently been applied to the synthesis of (BiPb).sub.2 Sr.sub.2 Ca.sub.2 Cu.sub.3 O.sub.x. However, the final material contained, in addition to the 2223, detectable amounts of 2212 plus a number of other compounds even after more than 60 hours of calcination.
Accordingly, it is an object of the present invention to provide a process for preparing BSCCO superconducting powders utilizing freeze-drying.
It is another object of the present invention to provide a process for preparing BSCCO superconducting powders such that phase separation is avoided and phase-pure Bi-2223 is obtained.
A further object of the present invention is to provide a process for preparing BSCCO superconducting powders having a substantially reduced processing time.